Communication and information devices often include electric circuits implemented on printed circuit boards (PCBs). These circuits can be affected by internal or external sources of noise that enter the electrical circuits by electromagnetic induction, electrostatic coupling, or conduction. This electromagnetic interference (EMI, also referred to as radio-frequency interference (RFI)) can degrade the performance of an electric circuit or, if the EMI is significant enough, prevent it from functioning altogether. The wireless communication bands around 900 MHz, 1.8 GHz, 1.9 GHz, 2.4 GHz, 5.8 GHz, and others are susceptible to EMI and can disrupt the operation of nearby electronic devices.
Filters can be used to mitigate EMI. For example, an EMI filter may include one or more resistors, capacitors, and/or inductors, where the values of the selected components determine the cutoff frequency of the EMI filter. Because discrete components such as resistors, capacitors, and inductors have manufacturing tolerances, it can be difficult and/or expensive to ensure that an implemented EMI filter has the intended cutoff frequency or stopband. Furthermore, discrete components are mounted to the surface of a PCB and therefore consume valuable surface area.
As an alternative to filters composed of resistors, capacitors, and/or inductors, or in conjunction with other components, ferrite beads may be used to attenuate or suppress EMI in electrical circuits, such as in PCBs. For example, a ferrite bead may be inserted to filter a DC power supply circuit that may carry unwanted EMI signals. The desired signals pass through the filter, and the undesired EMI is attenuated by the filter. The attenuation results from the frequency-sensitive impedance of the ferrite bead. Direct currents and low-frequency currents see only the conductor of the ferrite bead and are, therefore, unaffected. Higher-frequency energy couples with the ferrite bead, thereby developing impedance that has inductive and resistive components. When a signal trace passes through a ferrite core, low-frequency energy is transmitted with little loss, whereas higher-frequency energy encounters the inductive reactance caused by the real part of the complex permeability of the ferrite bead. The inductive reactance reduces the conducted EMI current and introduces an insertion loss, thereby improving (i.e., reducing) the circuit's sensitivity to EMI. Although ferrite beads can effectively combat EMI, ferrite beads tend to be expensive relative to resistors, capacitors, and inductors, and they result in additional series resistance in power traces. Moreover, ferrite beads are surface-mounted components and therefore, like other discrete components, consume valuable space on a PCB.
Therefore, there is an ongoing need for alternative ways to combat EMI in PCBs.